The Epidemiology Branch is conducting a number of birth defect studies in collaboration with the Health Research Board and Trinity College, Dublin, Ireland. The main objective of these studies is to determine the relationship between folate and birth defects. The birth defects studied to date are neural tube defects (NTDs), oral clefts, congenital heart defects and Down syndrome. These studies focus on biochemical factors in the area of folate metabolism, and on genetic mutations in folate related genes associated with birth defects. In the past we have shown that elevated homocysteine is a risk factor for NTDs, that a mutation in the methylenetetrahydrofolate reductase (MTHFR) gene 677C->T is a risk factor for both NTDs and oral clefts, and that a small dose of folic acid (100-200 micrograms) can raise red cell folate to levels that can prevent a fifth to almost a half of NTDs. Recently we showed that folic acid supplementation with as little as 100-200 micrograms daily can reduce homocysteine significantly. This is important both for NTD prevention and possible prevention of heart disease and cancer. We have just identified a new gene associated with increased risk for NTDs, methylenetetrahydrofolate reductase (MTHFD). This gene is important in the production of purine and pyrimidine for DNA synthesis. Mothers who have the R653Q variant of this gene are at increased risk of having a child with an NTD. This is the first maternal gene to be shown to be a risk factor for NTDs. Because of concerns that food fortification with folic acid could cause masking of vitamin B12 deficiency (deficiency with neurologic damage but without anemia), NICHD investigators also conducted a study to determine whether patients with vitamin B12 deficiency were more likely to present without anemia since fortification began. An increase in the proportion of patients presenting without anemia would be an indication that masking was occurring. We found that there was no significant change in the rate of B12 deficiency presenting without anemia since food fortification with folic acid began. We are currently collecting data and samples for genetic analysis on subjects with oral clefts and their families and samples from children with congenital heart defects and normal controls. We have samples from over 700 subjects with cleft lip or palate and from over 2000 children with congenital heart defects. These subjects will be studied to determine whether folate-related genes are etiologically important in clefts and congenital heart defects.